Public, Private, Not-for-Profit

How easy is it to have a portfolio career? Can you really have a foot in more than one camp and remain effective? Have you had any experience of crossing the above divides? Will some people ever get the meaning behind not-for-profit? Do you have a personal example of a related skills/career/work situation that others might learn from?

I have worked with people whose experiences have been all the more interesting for their diversification across some or all of these sectors. There is no definitive answer but the example of those who have done this successfully could also be viewed as individuals improving their definition of personal success and finding their work more meaningful as a result.

Any and all opinions are welcome!